William, unless this gun was special ordered from Marlin for 3" chambers, then it would have a stamp on the barrels water table. Ir would then have the standard 2 3/4" chambers, and I have seen some that have the 2 3/4" chambers on the same flat as the serial number. If it was opened by someone, I would still not shoot 3" loads from it. The guns at this time from Marlin were proofed at higher than nitro loads available at the time, and would be safe with the heavier loads offered THEN. This would not apply to today's modern Magnum 2 3/4" loads.
RWTF, If your Longrange is not stamped 3", then it is 2 3/4", unless someone opened it up. Not all Longrange or Wildfowl guns were 3" chambers.
My Longrange 3" chambered gun, the chamber gage goes to 2 15/16", and my gages are built from Hunter Arms Co. prints.
I'll bet you are right-but as i only shoot 2 & 3/4" shells in the aforementioned two 12 Smiths- and only RST 2.5" shells in my Specialty FWE circa 1933- Machts Nicht! If you want to shoot 3" shells, and I do for late season Winter pass shooting Canadas, then I use either one of two Model 12 heavy Duck guns, or my camo-ed- POS Mossy-Burgher 835 Utility-Mag- But NOT 3.5",I shoot one of those babies ONCE in the Mossy-Burgher 835- and I added some lead weight to the fiberglas hollowed butt-stock too (just as my M12 3" Mags have added lead weight in their butts as well) and I didn't much care for the recoil or the added muzzle plats through the portholes near the muzzle--
